1. Field of the Invention
The present invention relates to a charged particle beam drawing apparatus and a method of manufacturing an article by using the apparatus.
2. Description of the Related Art
Recently, as an electron beam drawing apparatus used to manufacture semiconductor integrated circuits, a multi-electron beam drawing apparatus designed to perform drawing by using a plurality of electron beams has been developed with miniaturization of drawing patterns. A multi-electron beam drawing apparatus turns on and off a plurality of electron beams to form pulse beams, and irradiates a substrate with the formed pulse beams. As disclosed in Japanese Patent Laid-Open No. 2007-128914, in order to perform high-accuracy drawing, it is necessary to calibrate the dose of an electron beam. The dose of an electron beam is obtained by multiplying the current of an electron beam by an irradiation time.
In the multi-electron beam drawing apparatus, one electron beam is a weak, high-speed pulse beam. As a beam detection device for detecting such a weak, high-speed electron beam, a detection device having the property of multiplying incident electrons, such as an Si photodiode, is used. As a preamplifier circuit using an Si photodiode for a multi-electron beam drawing apparatus, an integrating circuit is known. That is, as disclosed in Japanese Patent Laid-Open No. 2007-128914, a method of integrating a current i of an electron beam with respect to a pulse count N by using an integrating circuit is known. This method transfers the signal detected by a detection device to the integrating circuit on the subsequent stage and measures the signal integrated by the integrating circuit. The integrating circuit converts the charge obtained by integrating the current generated by the detection circuit into a voltage and hence is called a charge amplifier. Charge Q is expressed as temporal integration of a current i according to equation (1):Q=∫idt  (1)
Using an Si photodiode as a beam detection device results in electronically multiplying the current of an incident electron beam because the Si photodiode has an electronically multiplication effect. The irradiation time of a pulse beam is expressed by the time obtained by integrating the ON time of a pulse beam. Letting i be the current of an electron beam incident on the beam detection device, G be the electron multiplication gain of the beam detection device, and T be the sum total of ON times of electron beams, the charge Q output from the beam detection device and accumulated in the charge amplifier is expressed by equation (2).Q=i×G×T  (2)
The electron multiplication gain G of the beam detection device changes depending on the energy of electrons (accelerating voltage) incident on the Si photodiode. The multiplication gain is disclosed on a data sheet or research paper published by the maker of Si photodiodes. FIG. 3 shows an example of the arrangement of a charge amplifier 118. An operational amplifier 31 in use is a high-accuracy, high-band amplifier having properties such as high band, low noise, and low offset. A feedback condenser (capacitor) 32 has an electrostatic capacitance Cf and accumulates small charge obtained by integrating the current output from a beam detection device 112. A reset switch 33 discharges the charge accumulated in the feedback condenser 32. A distributing cable has an electrostatic capacitance Ci. In general, a coaxial cable is used as this cable. Depending on the type of coaxial cable to be used, its electrostatic capacitance is determined by specifications. For example, an RG/174/U coaxial cable has an electrostatic capacitance of 101.0 pF/m. The longer the cable, the larger the electrostatic capacitance. An input voltage is mainly a noise voltage with a voltage value Vi. The charge amplifier 118 outputs a voltage Vo.
The charge amplifier 118 uses the condenser 32 having the small electrostatic capacitance Cf as the feedback circuit of the operational amplifier 311, and converts input charge Qi into the voltage Vo. The relationship between these values can be expressed by equation (3):Qi=Cf·Vo  (3)
The following is a numerical example. This numerical example is written as an example in terms of calculation. Assume that the current i of an input beam is 50 pA, the electron multiplication gain G of the beam detection device 112 is 1000, and a beam ON time T is 100 μs. The numerical example is substituted into equation (2). As a result, Qi becomes 5 pC. In this case, if the electrostatic capacitance Cf of the feedback condenser 32 is 1 pF, an output of V0=5 V can be obtained from the charge amplifier 118 according to equation (3). In addition, in order to obtain a beam ON time of 100 μs, it is necessary to apply an electron beam 2,000 times, assuming that the ON/OFF period of a current is 10 MHz and the duty ratio is 50%. Using a detection device having no multiplication effect on the current of an electron beam such as a Faraday cup will make the input charge Qi of the charge amplifier 118 become 5 fC (femtocoulomb) even if the beam has the same current value as that described above. As a result, the voltage output from the charge amplifier 118 is 5 mV. As the current value of the electron beam further decreases, the output voltage Vo decreases and is buried in noise unless the electrostatic capacitance Cf of the feedback condenser 32 is reduced to a smaller value. As a result, the S/N ratio of the output excessively decreases to make it impossible to obtain necessary detection precision. In the charge amplifier 118, charge stays in a parasitic capacitance such as the electrostatic capacitance Ci generated in a wiring such as a cable or noise is generated when vibration acts on the cable.
As described in Japanese Patent Laid-Open No. 2007-128914, as the length of the distributing cable between the beam detection device 112 and the charge amplifier 118 increases, the parasitic capacitance error increases. In a multi-electron beam drawing apparatus designed to perform drawing by individually turning on and off n electron beams at high speed, the measurement time increases n times that in the case of a single beam. It is therefore necessary to shorten the measurement time. It is possible to shorten the integration time by decreasing the capacitance value of the condenser used for the integrating circuit to a small value and increasing the amplification component of an output voltage relative to an input voltage. When a movable stage incorporates the beam detection device 112 such as an Si photodiode, the state of the wiring between the beam detection device 112 and the charge amplifier 118 changes. In addition, since the amplification factor of the beam detection device 112 changes depending on the accelerating voltage of the electron source, it is difficult to apply constant charge to the input with predetermined charge conditions.
In the multi-electron beam drawing apparatus, since the measurement time is very long as compared with the case of a single beam, it is necessary to shorten the measurement time. In the above concrete numerical example, when obtaining the same output voltage, that is, 5 V, from the charge amplifier upon changing the beam ON time to 10 μs, the value of the feedback condenser of the charge amplifier becomes a very small capacitance value of 0.1 pF. Such a condenser with a small electrostatic capacitance is susceptible to the influence of circuit implementation and a parasitic capacitance such as a cable electrostatic capacitance. If the electrostatic capacitance of the condenser of the charge amplifier is not calibrated while the charge amplifier is mounted on the apparatus, it is not possible to accurately measure the current of an electron beam. This leads to a failure of properly calibrating the dose of an electron beam.
Japanese Patent Laid-Open No. 63-294254 discloses a constant charge generating apparatus which calibrates a charge amplifier. This constant charge generating apparatus is an apparatus which holds charge in a condenser and supplies charge to a charge amplifier. In this case, a change in the capacitance of the condenser makes it impossible to obtain a correct measurement value. In addition, the condenser which supplies charge is a condenser having a small capacitance value. In addition, a condenser which supplies charge and the feedback condenser of the charge amplifier are condensers having small capacitance values. Even when a small condenser having several pF is mounted on a circuit board, the capacitance value changes depending on the wiring pattern used. It is therefore necessary to calibrate the signal generator itself which performs calibration.